Continuous separation processes for the selective adsorption of one component or group of components from a mixture are common in industry. Examples of such processes are the separation of linear paraffins from branched-chain and cyclic hydrocarbons, clefins from paraffins, para-cresol or meta-cresol from cresol isomers, para-cymene from oymene isomers, 1-butene from a mixture of paraffins and clefins which contain four carbon atoms, fructose and glucose from mixtures thereof, para-xylene from xylene isomers, ethylbenzene from aromatic isomers containing eight carbon atoms, cyclic hydrocarbons and olefins from paraffins and others.
Generally, such separation processes use a solid adsorbent which preferentially retains the component or group of components of interest in order to separate them from the rest of the mixture. There are a wide variety of solid adsorbents available, and each separation application may require a different solid adsorbent. For example, the separation of linear paraffins from branched-chain or cyclic hydrocarbons typically requires a molecular sieve commonly known as 5A, while another application would require a completely different adsorbent. Often, the solid adsorbent is in the form of a simulated moving bed, where the bed of solid adsorbent is held stationary, and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds, and different applications may require differing numbers of sub-beds. The shift in the locations of liquid input and output in the direction of the fluid flow through the bed simulates the movement of the solid adsorbent in the opposite direction. Commercially, moving the locations of liquid input and output is accomplished by a fluid directing device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time, the rotary valve advances one index and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to a new valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time is uniform for each valve step in a valve cycle, and ranges generally from about 35 to about 200 seconds. A typical process contains from 8 to 24 adsorbent sub-beds, an equal number of distributors located between the each adsorbent sub-bed, two liquid input lines, two liquid output lines, and associated flush lines.
The principal liquid inputs and outputs of the adsorbent system consists of four streams: the feed mixture, the extract, the raffinate, and the desorbent. Each stream flows into or out of the adsorbent system at a particular flow rate, and each flow rate is independently controlled. The feed, which is introduced to the adsorbent system, contains the component or group of components which are to be separated from other components in the feed stream. The desorbent, which is introduced to the adsorbent system, contains a liquid capable of displacing feed components from the adsorbent. The extract, which is withdrawn from the adsorbent system, contains the separated component which was selectively adsorbed by the adsorbent, and desorbent liquid. The raffinate, which is withdrawn from the adsorbent system, contains the rest of the feed components which were less selectively absorbed by the adsorbent, and desorbent liquid. There also may be associated flush streams introduced to and withdrawn from the adsorbent system. The four principal streams are spaced strategically throughout the adsorbent system and divide the sub-beds into four zones, each of which performs a different function.
Zone I contains the adsorbent sub-beds located between the feed input and the raffinate output, and adsorption of the component of interest takes place in this zone. Zone II contains the adsorbent sub-beds located between the extract output and the feed input, and the desorption of components other than those of interest takes place in this zone. Zone III contains the adsorbent sub-beds located between the desorbent input and the extract output, and the component of interest is desorbed in this zone. Finally, Zone IV contains the adsorbent sub-beds located between the raffinate output and the desorbent input, and the purpose of this zone is to prevent the contamination of the component of interest with other components.
The other two important separation process streams for the purpose of this invention are the pumparound and pusharound streams. In typical separation processes the adsorbent beds are housed in chambers; usually ranging from 1 to 24 or more chambers. For example, if the separation process contains 24 sub beds which are split into two chambers, one chamber would contain sub-beds 1 through 12 and the other would contain sub-beds 13 through 24. Although functionally the adsorbent system as a whole does not have a top or a bottom, each chamber has a physical top and bottom. The pumparound and pusharound streams each conduct the liquid effluent exiting the physical bottom of one adsorbent bed chamber back up to reenter the physical top of the other adsorbent bed chamber. In the 24 sub-bed example, the pumparound stream would be the stream that conducts the effluent of sub-bed 24 from the physical bottom of the second chamber to reenter sub-bed 1 at the physical top of the first chamber, and the pusharound stream would conduct the effluent of sub-bed 12 from the physical bottom of the first chamber to reenter sub-bed 13 at the physical top of the second chamber. It may be possible, however, that the separation process contains only one adsorbent chamber and has only a pumparound stream. It is important to note that the composition of the pumparound or pusharound stream changes with each valve step, and in one valve cycle both streams will have sequentially carried the composition which corresponds to each valve position.
The foregoing is a brief description of relevant portions of the a separation process; for a more detailed explanation, see Mowry, J. R. In Handbook of Petroleum Refining Processes; Meyers, R. A. Ed.; McGraw-Hill: New York, 1986; pp 8-79 to 8-99. For greater detail regarding the simulated moving bed and its operation, see U.S. Pat. No. 2,985,589.
The common practice in industry is to control separation processes either by on-line gas chromatography analyses, or by off-line laboratory analyses. When controlling on-line, the gas chromatography analysis of the pumparound stream generally requires about 10 minutes which is considerably greater than the usual step time of the rotary valve. Therefore, only select valve positions may be sampled and analyzed. Generally, only Zone II near the extract output and Zone IV near the desorbent input are sampled and analyzed. The data provided by this on-line gas chromatography procedure is useful for detecting process upsets, but unfortunately analyzing the composition of only two valve positions provides limited information regarding the performance of the separation process and is only minimally useful for precise separation process control.
A more thorough control is accomplished using off-line laboratory gas chromatography or high performance liquid chromatography analyses to determine the values of the concentrations of the components in samples of the pumparound stream taken at each valve position in a valve cycle. The measured concentrations are then plotted versus their relative valve positions to form what is generally called a profile. Using the profile, the recovery and purity of the component of interest can be calculated and the degree of optimization of the separation assessed. Then required changes in the step time and liquid stream flow rates may be determined and implemented. The drawbacks to controlling a separation process in this fashion are the time delay between sampling and analytical results where the latter are used to determine whether or what changes should be made, the labor involved to manually collect the stream samples, and the personal exposure of the operator manually collecting the stream samples. Since the analyses are performed off-line, the time delay may be from one to several days long. Because of the drawbacks, refiners generally only perform this procedure about once every six months or if there is a problem with the separation process.
Other separation processes have been controlled using spectroscopic determinations of impurities in the separated pure product. For instance the Canadian Patent Application 2,050,108 discloses spectroscopically measuring one component of a mixture in another component of the mixture following the separation of the mixture into its components. The results of the measurements are used to control the separation so that the amount of impurity in the pure product is controlled to a desired value.
The present invention moves beyond the current practice and discloses a more useful process of control through conveniently providing the necessary quantity of information for precise process control in a far more beneficial time frame. Specifically, the present invention allows for on-line virtually instantaneous measurements of the values of the components at each valve position, which in turn allows a complete profile of the separation to be generated in one valve cycle. Adjustments to the separation process may then be timely made, and the para-xylene purity and/or recovery may be controlled with efficiency, precision, and accuracy. Furthermore, the control process may be fully automated so that the greatest amount of information is timely and conveniently available.